Ceramic dielectric waveguide filters are well known in the art. In the electronics industry today, ceramic dielectric waveguide filters are typically designed using an “all pole” configuration in which all resonators are tuned to the passband frequencies. With this type of design, one way to increase the attenuation outside of the passband is to increase the number of resonators. The number of poles in a waveguide filter determines important electrical characteristics such as passband insertion loss and stopband attenuation. The length and width of the resonant cavities, also known as resonant cells or resonators, help to set the center frequency of the waveguide filter.
U.S. Pat. No. 5,926,079 to Heine et al. shows a prior art ceramic dielectric monoblock waveguide filter in which five resonators are spaced longitudinally in series along the length of the monoblock and an electrical signal flows through successive resonators in series to form a passband. Waveguide filters of the type disclosed in U.S. Pat. No. 5,926,079 to Heine et al. are used for the same type of filtering applications as traditional dielectric monoblock filters with through-hole resonators of the type disclosed in, for example, U.S. Pat. No. 4,692,726 to Green et al. One typical application for waveguide filters is use in base-station transceivers for cellular telephone networks.
It is also well known in the art that the length and width of a ceramic waveguide filter such as, for example, the ceramic waveguide filter disclosed in U.S. Pat. No. 5,926,079 to Heine et al., defines and determines the passband frequency of the waveguide filter while the height/thickness of the waveguide filter determines the unloaded “Q” of the waveguide filter resonators and therefore the insertion loss in the passband of the waveguide filter. In U.S. Pat. No. 5,926,079 to Heine et al., the positioning of blind input/output holes centrally in monoblock bridge regions formed between the resonators and in a relationship adjacent slots defined in the monoblock provide the necessary external coupling bandwidth of the waveguide filter.
The plating of blind input-output holes during the manufacturing process however has proven unreliable and can lead to unpredictable filter performance. The use of plated input/output through-holes has proven satisfactory in certain applications including, for example, the relatively thin resonators of waveguide delay lines of the type disclosed in US Patent Application Publication No. 2010/0024973. However, coupling with plated input/output through-holes, when used with thick waveguide filters, limits the external bandwidth to unduly narrow band filters.
The present invention is thus directed to a new and novel structure and method for providing the necessary external bandwidth in a thick waveguide filter which includes plated input/output through-holes without an increase in the insertion loss of the waveguide filter.